1. Field of the Invention
The present invention relates to a display apparatus of matrix type, for example, a matrix type liquid crystal display apparatus which has large capacitance which can be used for AV (Audio-Visual) instruments, OA (Office Automation) instruments, computers and the like.
2. Description of the Related Art
With advance of a highly computerized society, a request for a larger display having larger capacitance has been rising. To comply with the request, a cathode-ray tube (CRT), which is called as "a king of displays", is being developed to be more fine. As to the size of the CRT, the maximum size of a direct-view type CRT is 40 inches and the maximum size of a projection type CRT is 200 inches. The CRTs known by the present inventors, however, are too heavy and long in depth to prevent the realization of a large display having larger capacitance. To avoid the shortcomings the CRTs entails, a drastic solution has been desirous.
A planar type display, which is operated on the different principle from the known CRTs, is now under steady study toward the higher quality stage for a highvision or an engineering workstation (EWS) from the current way of use for a word processor or a personal computer.
The planar type display includes an electroluminescent panel (ELP), a plasma display panel (PDP), a fluorescence character display tube (VFD), an electro-chromic display (ECD), and a liquid crystal display (LCD). The most promising planar type display among those displays is the liquid crystal display because it has the shortest way to the realization of full color and the most suitable characteristics to an LSI (Large-Scale Integration). Hence, the liquid crystal display is remarkable in technical progress.
In recent days, a matrix type liquid crystal display apparatus is more likely to be requested to have larger capacitance. That is, with the recent request for higher resolution of display apparatuses, it has been requested that the number of pixels is increased from 400.times.600 to 1000.times.1000 or more and the size of the screen is also increased from 10 inches to 20 inches or more.
The present inventors know that the matrix type liquid crystal display apparatus (referred to as a matrix type LCD apparatus) is, in large, divided into an active matrix driven type liquid crystal display (referred to as an active matrix type LCD) and a simple matrix driven type liquid crystal display (referred to as a simple matrix type LCD). The division standard depends on the difference of the driving method therebetween.
The simple matrix type LCD includes the structure wherein liquid crystal is enclosed in an XY matrix panel composed of a pair of glass substrates located in opposition in a manner to cross the striped electrodes formed on one of the glass substrates with the striped electrodes formed on the other of the glass substrates at right angles, and is operated to display through the effect of sharpness of liquid crystal display characteristic. The active matrix type LCD includes the structure wherein non-linear elements are directly added to pixels and are operated to display through the effect of the non-linear characteristic of each element (switching characteristic, and the like). As compared to the simple matrix type LCD, the active matrix type LCD has lower dependency on the display characteristics of the liquid crystal itself, so that it can realize a high-contrast and high-response display. This kind of non-linear elements are divided into a two-terminal type and a three-terminal type. The two-terminal type non-linear element is MIM (Metal-Insulating material-Metal) or a diode. The three-terminal type non-linear element is TFT (Thin Film Transistor), Si-MOS (Silicon Metal Oxide Semiconductor) or SOS (Silicon-On-Sapphire). For each of the LCDs, the study for enhancing the resolution and enlarging the screen has been made.
Both the active matrix type LCD and the simple matrix type LCD are disadvantageous to realization of a large screen display having large capacitance in light of contrast, a response speed, and reliability.
For the active matrix type LCD, in particular, a thin film transistor (referred to as TFT) active matrix type LCD, some problems to be mentioned below still exist in the study for enhancing the resolution and enlarging the screen.
As the scan lines are increased in number, the writing time for one scan line is decreased. It results in needing larger current for properly driving TFT elements. In order to increase the on current, it is necessary to use a semiconductor material having large mobility for composing the TFT element or to enhance a W/L (Width/Length) ratio of the TFT element. The former case concerns with the property of the semiconductor material, which it is difficult to greatly improve. To implement the latter case, it is necessary to control the manufacturing process of the TFT element very finely, which may result in greatly lowering the yields of the TFT element.
If an area ratio of a TFT element to a pixel is made larger as the resolution becomes higher, the capacitance between a gate and a drain of the TFT element is made larger than the capacitance of the liquid crystal. This results in allowing a gate signal to have a large adverse effect on pixels.
That is, since the active matrix type LCD makes a positive use of a switching characteristic and a nonlinear characteristic of a nonlinear element for displaying an image, the degradation of the display resulting from the realization of larger capacitance is not so remarkable as the simple matrix type LCD. In actual, since the parasitic capacitance exists in the nonlinear element through the scan lines, the problems such as a lower contrast, an after image, and a shorter life of a panel may appear which are caused by a leakage of a scanning electric signal to pixel electrodes. As to the size of the screen, since the wires are made longer, the connection of the increase of wire resistance and the parasitic capacitance results in bringing about attenuation on signal lines, thereby giving a large adverse effect on the evenness and the contrast of the display. It means that this type of LCD earnestly needs an epoch-making novel technique for realizing the larger LCD having large capacitance.
Turning to the simple matrix type LCD, some problems to be mentioned below still exist in the study for enhancing the resolution and enlarging the screen.
As the number of the scan lines is increased more, it is difficult to keep a voltage ratio of selective pixels to non-selective pixels large, resulting in bringing about some shortcomings such as a lower contrast, a smaller visual area and a slower response. Those shortcomings greatly degrade the quality of the display. In actual situations, the critical duty factor is about 1/200 to 1/400. To overcome the appearance of such shortcomings, an upper and lower divisional driving system has been proposed. This system is composed by dividing the signal lines formed on the panel display into an upper and a lower blocks and individually scanning the upper and lower display blocks. Concretely, if not divided, the number of the scan lines is 400, while if divided, the number of the scan lines is apparently 800. The system at the trial stage reaches the display capacitance of 800.times.1024 lines. This system makes it possible to scan 2N scan lines at a 1/N duty factor, so that the number of the scan lines is apparently doubled. This system, however, has some problems as well. For example, it requires twice as many driving circuits as the simple matrix type LCD without using this system. Further, it is incapable of scanning 2N or more scan lines.
That is, in the simple matrix type LCD, a ratio of an effective voltage of selective pixel electrodes to non-selective pixel electrodes comes closer to 1 as the scan lines are increased in number. The display characteristic of the liquid crystal itself is required to have sharpness. The securable sharpness is limited. In actual, the number of the scan lines is suppressed to be about 400. The response speed is likely to be opposed to the sharpness of the display characteristic and is made slower as the number of the scan lines (the number of duties). In general, when the number of the scan lines is 400, the response time is 100 ms (millisecond) to 300 ms or some.
The foregoing upper and lower divisional driving system needs two drivers for the data signal lines for the upper and the lower blocks, resulting in providing twice as many as the normal simple matrix type LCD, thereby making the LCD costlier if this divisional system is used. Moreover, the divisional system is inferior to the active matrix type LCD in light of contrast and response speed. The wires of the transparent electrodes are made longer as the display panel is made larger. It results in increasing the wiring resistance, thereby disadvantageously lowering the display unevenness and contrast resulting from the attenuation of the signals. It means that a breakthrough technique is necessary to the realization of the larger display having larger capacitance.